In the case where a cryogenic propellant is kept at a temperature below its boiling point but above its freezing point, the propellant is considered sub-cooled. This is typically done to increase the density of the propellant and therefore increase the amount of mass that can be pumped assuming a constant volume flow rate.
Vaniman, J. L., A. L. Worlund, and T. W. Winstead. "Slush and subcooled propellants for lunar and interplanetary missions." In Advances in Cryogenic Engineering, pp. 20-29. Springer, Boston, MA, 1969.